My #1 utilized software tool is email. A few years back I checked the stats for the last year for my work and personal email combined: I received and read on average 397 emails a week, and sent 358 emails a week that year (this excludes the email which is spam or which I have auto-filters to file to certain folders -- these are real emails I read, write and process).
Over the years I've adopted some best practices to process and write email -- I hope you find some of them useful.